1. Field of the Invention
This invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic field gradient coil system suitable for use in a magnetic resonance imaging apparatus of superconducting type.
2. Description of Related Art
In a magnetic resonance imaging apparatus, a pulse current is supplied to its magnetic field gradient coil unit. Thus, when an electrical conductor is present in a range which is influenced by the magnetic field produced by the magnetic field gradient coil unit, an eddy current is induced in the electrical conductor at the rise time and fall time of each of the pulses of the pulse current. The flow of this eddy current leads to a decreased field strength of the effective magnetic field gradients produced by the magnetic field gradient coil unit and leads also to a degraded linearity of the magnetic field gradients.
In a magnetic resonance imaging apparatus of superconducting type using a superconducting coil unit kept at a very low temperature, a heat reflecting cylindrical member is provided so as to enhance the effect of thermally shielding the superconducting coil unit against the ambient atmosphere, and this heat reflecting cylindrical member is located to surround a magnetic field gradient coil unit. Leakage of the magnetic field produced by the magnetic field gradient coil unit toward the superconducting coil unit should be avoided, and, for that purpose, the heat reflecting cylindrical member is made of an electrical conductor such as aluminum or copper, so that flow of an eddy current in the heat reflecting cylindrical member can produce an inverse magnetic field which cancels the leakage magnetic field.
The superconducting coil unit is disposed inside a coolant tank filled with a coolant such as liquid helium, and this coolant tank is commonly made of a stainless steel having a high electrical resistance. Therefore, when the magnetic field produced by the magnetic field gradient coil unit leaks toward and into the coolant tank, an eddy current is induced in the stainless steel forming the coolant tank, and the temperature of the coolant tank rises up to a high level so as to accelerate vaporization of the coolant. This is the greatest reason why leakage of the magnetic field produced by the magnetic field gradient coil unit toward the superconducting coil unit should be prevented.
However, as described already, the flow of the eddy current in the heat reflecting cylindrical member leads to a decrease in the primarily required strength of the effective magnetic field gradients produced by the magnetic field gradient coil unit and leads also to a degraded linearity of the magnetic field gradients. It is now strongly demanded to reduce the size of the superconducting magnet. However, when the size of the superconducting magnet is to be made smaller, the heat reflecting cylindrical member must inevitably be located closer to the magnetic field gradient coil unit. Thus, when the heat reflecting cylindrical member is located at such a position, the adverse effect of the eddy current will be further intensified, and the desired reduction of the size of the superconducting magnet will become impossible as a matter of fact.
Journal of Physics E: Scientific Instrument, 1981, Vol. 14 discloses, at pages 876-879, a magnetic field gradient coil system for forming a magnetic field gradient in a direction of a Z-axis of an X, Y, Z orthogonal coordinate system having its origin at about the center of a sensor probe, the magnetic field gradient coil system comprising one pair of inner coils located in a relation symmetrical with respect to an XY plane having its center on the Z-axis and including the origin, and one pair of outer coils located also in a relation symmetrical with respect to the XY plane, the inner coils having the same radius a and being disposed at a distance d.sub.1 from the origin, while the outer coils having the same radius as that of the inner coils and being disposed at a distance d.sub.2 from the origin. In the disclosed magnetic field gradient coil system, the values of the distances d.sub.1 and d.sub.2 are selected to be .vertline.d.sub.1 .vertline.=0.49a and .vertline.d.sub.2 .vertline.=1.28a when the ratio b/a between the mean radius b of a cylindrical electrical conductor forming a heat reflecting cylindrical member and the radius a of the inner and outer coils is selected to be b/a=9/7.
According to the disclosed magnetic field gradient coil system, the axial magnetic field component of fifth order shows a great decrease. However, because the axial magnetic field component of third order is not cancelled by the inner and outer coils, the effective magnetic field gradient in the direction of the Z-axis has a degraded linearity, and the field strength of the effective magnetic field gradient shows also a great decrease. It has been clarified that the degraded linearity and decreased field strength are attributable to the following facts:
(1) Although the value of the axial magnetic field component of fifth order is nearly zero in the case of the inner coils, the axial magnetic field component of first order does not show its maximum value.
(2) Although the value of the axial magnetic field component of fifth order is nearly zero in the case of the outer coils, the axial magnetic field component of first order shows a very small value.
JP-A-58-53742 discloses a magnetic field gradient coil system which comprises one pair of inner coils and one pair of outer coils similar to those described above and which improves the linearity of the magnetic field gradient. However, in the latter magnetic field gradient coil system which is associated by a heat reflecting cylindrical member made of an electrical conductor, the adverse effect of an eddy current induced in the electrical conductive cylindrical member is not taken into consideration at all.